what type of transmission is it? that's the most important piece of information here...it should say what model it is in your owner's manual. http://www.ehow.com/how_5027137_dete...on-gm-car.html
use this link if you need help finding out your gearbox model. if you have any transmission/clutch problems currently, that may immediately disqualify it from being bolted on a higher-torque V8. if these problems exist, they should be taken care of before you do an engine swap. a junkyard gearbox may also be a good option for you. i'd recommend joining a silverado forum, where they will have many more members in this area of expertise.